When directing backups from multiple policies to a single volume, you should make sure that these backups have similar backup frequency and retention periods. Even though backups expire based on their retention period, the space consumed by these backups in the Snapshot copy might not be released until all the other backups (from different policies) in the same Snapshot copy have expired.
Archival Snapshot copies are taken for the entire volume. A volume can contain backups from multiple policies, which means that backups from different policies can be captured in the same archival Snapshot copy. SnapVault for NetBackup will remove an existing archival Snapshot copy only after all of its captured backups reach the end of their retention periods. The backup with the longest retention period therefore dictates when the archival Snapshot copy will be deleted. The space allocated for the other backups will continue to be consumed until the Snapshot copy is deleted.
Let us consider two policies, PolA and PolB. The retention period for PolA is one year and the retention period for PolB is three months. After a full backup of both these policies, there is one archival Snapshot copy, SnapX, which captures the backup from both of these policies. SnapX is deleted (thereby reclaiming the allocated storage space) only the backups from both PolA and PolB expire. This can result in less than optimal storage savings. In this case, the space allocated for backup for PolB will be retained for one year even though it is expected to be kept for only three months. This occurs because both PolA and PolB share a common archival Snapshot copy, SnapX, which will get deleted only after the expiry of backups from both PolA and PolB. To avoid this issue, you should direct only backups with similar backup frequency and retention to the same volume.